Simulations of the effects of interannual and decadal variability on the clear‐sky outgoing long‐wave radiation spectrum

Using atmospheric profiles derived from the Hadley Centre atmosphere climate model version 3 (HadAM3) as input to a radiative transfer code, the sensitivity of the resolved spectrum of clear‐sky outgoing long‐wave radiation to both interannual and longer‐term atmospheric variability has been analysed. A comparison of the simulated spectra with available observations from two satellite‐based instruments indicates a reasonable match, although consistent differences are present. These may be explained by a combination of uncertainties in the atmospheric state, and in the relative calibration of the two instruments. Focusing on the simulations: if HadAM3 is forced by the observed sea surface temperature (SST) record alone, and long‐term alterations in the well‐mixed greenhouse gases are imposed in the radiance simulations, the changes seen within the major absorption bands are robust. Under a second scenario, where the effects of solar variability, volcanic aerosol, ozone changes and increases in the well‐mixed greenhouse gases are also included in the forcing of HadAM3, the long‐term profile changes tend to show an enhanced upper‐tropospheric warming and low/mid‐stratospheric cooling, with increased near‐surface humidities compared to the SST‐only case. However, the tropospheric response of the system, manifested in the spectral change pattern over the atmospheric window and water vapour bands, falls within the range of year‐to‐year variability. Copyright © Royal Meteorological Society, 2003. R. P. Allan's contribution is Crown copyright.